There have been many studies published over the years highlighting the fact that central corneal thickness has a significant impact on intraocular pressure (IOP) measurements obtained by tonometry (specifically Goldmann contact tonometry in the OHTS). Perhaps most significant was the Ocular Hypertension Treatment Study (OHTS). At the simplest level, a thicker cornea is likely to offer greater resistance to any force attempting to deform the corneal surface. It is expected, therefore, that where two patients have been measured as having similar IOPs by conventional tonometry means (either contact or non-contact applanation tonometry), and also found to have different corneal thicknesses, the actual IOP will be different between the two eyes. It is also well known that patients are found to have lower conventionally obtained IOP measurements after refractive surgery than before, the thinner corneas having less resistance to the tonometer force. For this reason, many clinics will give patients information cards showing their preoperative IOP measurements so that long-term IOP assessment throughout the time before and after surgery may be put into context. In the light of this significance, most glaucoma clinics when deciding to prescribe ocular hypotensives will require a pachymetry reading to assess the actual IOP level. Many instrument manufacturers are now also supplying pachymeters to hospitals and to optometrists along with conversion charts to allow an IOP reading to be adjusted relative to a standard corneal thickness and therefore ensure that all clinical decisions are made to a standardised protocol. It is likely that corneal thickness assessment will become a standard part of the optometrist routine in years to come. Many researchers have begun to question the rather clear-cut assumption that corneal thickness is the main determinant of tonometry reading variation between patients with similar IOPs. While the corneal thickness is certainly an important factor in the corneal resistance offered to an applied force, other factors, such as state of hydration or curvature of the cornea, are also influential and may affect the IOP value obtained by conventional tonometry. A new instrument, the Reichert Ocular Response Analyzer (ORA) uses a non-contact applanation mechanism but measures the corneal response to the force applied beyond the point of applanation and the return to normal curvature. This allows it to measure what has been termed corneal hysteresis and claims therefore to offer IOP measurements independent of corneal variables and, potentially, allow identification of certain corneal conditions (such as ecstatic states like keratoconus).